Everyone always wants to know what the best free is instead of which one works the best out of all. Simply put, As I have said a million times, No such thing as a best free anything. Especially anti virus. Like with most other things in this world, You get what you pay for.
But, I will answer your question. You may not be able to afford paid. If you can't, If it were me, I would go with Avast. MSE just isn't that good. MSE just lets to much get by that it really shouldn't. But, Don't expect great works from Avast alone. It has it's faults to. Trojan Mal ware and drive by downloads. Avast has real time protection. But, It's not good enough. If I were going to use it, It would be with the paid version of Mal ware bytes. Mal ware bytes paid version has real time protection. The free version doesn't. The paid version will enhance Avast free version making it harder to break through. Use the Comodo fire wall or if you have Windows 7 or Vista, Use the firewall that came with the running system. It's just as good as any free one. With XP, Use Comodo fire wall.